Large Mono Coil Shapes

[bbsailor]

My first PI detector design (1986) was designed for divers. It was running at 3000 PPs and had a delay of 30us. In the water there was no need for shielding.
Recent designs with 10,000 PPs and a delay of 3us are much more sensitive. Without shielding they see my hand as a target.
My hand is full of blood and tissue that are conductive. So I consider it normal that the detector sees it as a target.

However, I live in a region with high static during dry weather. The static can be strong enough for sparks to fly from my fingers. This means several thousands of volts.
I design the shielding for this static and test it with a PVC pipe which I charge by rubbing with a cloth until I hear the crackling of the static electricity discharging and it picks up pieces of plastic or styro foam.
If I can approach this charged PVC pipe to the detector coil without noticing any reaction from the detector, I consider the coil well shielded.
I notice that the shielded coils reduce environmental EMI to a great extend.
The coil shielding adds capacitance to the coil and therefore reduces the SRF. Leaving a minimum of space of 3mm between the coil wires and the shield helps. Leaving 10mm of space reduces the capacitance a lot.

Here are a few more things to consider when optimizing a coil shape and size for a target.

1. Target time constant. A fully stimulated target will have its TX activated eddy currents decay by 95 percent in 3 time constants. If you are seeking a low TC target with a 1 micro second TC you will need a very low delay when switching from TX turn off to RX turn on. Getting a low delay relates to the next point.
2. Low coil seen capacitance. The coil sees the following characteristics that affect coil capacitance.
A. Coil insulation dielectric constant with 2 being low.
B. Coil insulation thickness with dry air between coil wire turns being near 1.
C. MOSFET coil driver capacitance minimized by using a series diode to the coil.
D. Damping resistor value with higher values representing less capacitance to damp and potentially a lower delay.
E. Coil to shield capacitance affected by the area of the shield to the coil wire bundle separated by the space between the coil and dielectric constant of the spacer material. Here is where I found that using Scotch24 wire mesh shield adds less capacitance than a solid shield material.
F. Coax capacitance between the coil to the connection point to the active circuits. Use shortest coax and lowest capacitance coax.
3. Coil parts seen as a target at low delays.
A. Coil wire. Review the skin effect of your chosen coil wire type, solid or stranded and wire gauge.
B. Any solder joints inside or near the coil.
4. Coil diameter and shape relative to target size and time constant.
A. Round coils have the most inductance for a coil perimeter.
B. Oval coils or round coils made thinner into a more compressed oval shape will have less inductance as the oval is made thinner and wider.

The above things are variables that can begin to interact with each other based on:
1. Target size.
2. Target time constant.
3. Lowest operational delay.
4. Coil size and shape.
5. Target depth desired.
6. TX pulse rate
7. Any RX integration techniques being used and number of RX pulses being integrated.

Joseph J. Rogowski

[GeoMax]

Hi BBSailor

Question, was the German physicist Walter Rogowski (1881-1947), the inventor of the Rogowski coil a relative from you?

GeoMax

[bbsailor]

Hi BBSailor

Question, was the German physicist Walter Rogowski (1881-1947), the inventor of the Rogowski coil a relative from you?

GeoMax

Not that I know of

All of my coil knowledge came from understanding inductance theory, pulse induction optimum TX current rise, TX turn off discharge slope and it’s ability to stimulate a target, the coil seen capacitance as related to the damping resistor value and finally how these variables interact with different mono coil designs. By winding many coil sizes and shapes, taking accurate inductance measurements, observing coil self resonant frequencies and target detection distances with different time constant targets, I began to see how these things interact.

Once you do enough of this type of activity, you too will begin to build a good mental model and know what coil design tradeoffs you need to make when you are seeking a particular time constant range of targets.

Playing the guitar and working with a custom guitar maker in Newark NJ got me started with winding coils for guitar pickups while in high school.

Joseph J. Rogowski

[GeoMax]

Since this is a topic about large mono coil shapes, the following question fits to this topic.

Usually big frame coils are designed for deep large targets. Those coils are not very sensitive to small targets and usually carried by two people. Right?
Would it make sense to build inside the big coil frame a smaller coil frame to catch additional small targets as well? The coil arrangement would be like
a dual field coil (see image below).

What is your opinion to this idea? Would it make sense?

[bbsailor]

[QUOTE=GeoMax;296209]Since this is a topic about large mono coil shapes, the following question fits to this topic.

Usually big frame coils are designed for deep large targets. Those coils are not very sensitive to small targets and usually carried by two people. Right?

Depending on the size and weight of the coil.

Would it make sense to build inside the big coil frame a smaller coil frame to catch additional small targets as well? The coil arrangement would be like
a dual field coil (see image below).

What is your opinion to this idea? Would it make sense?

First you need to create some input data to even begin to think about this question.




1. What is the size, depth and time constant of your desired large targets?
2. What is the TX frequency, coil resistance, coil inductance, coil diameter or perimeter of the larger coil?
3. What is the size, depth and time constant of your desired small targets?
4. What is the TX frequency, coil resistance, coil inductance, coil diameter or perimeter of the smaller coil?

Is this dual coil design practical due to how you switch between the larger coil and the smaller coil?
1. Mechanical switching?
2. Electronic switching?
3. Alternating between two coils while searching with an indicator which coil detected a target?
4. Selecting TX frequency parameters to fully stimulate large targets without the smaller coil looking like a target?

Unless you answer these questions, build a prototype, test for and observe the interaction of the the inner coil size, shape and location, you will not get a good answer to your question.

Read this: https://www.geotech1.com/pages/metde.../induction.pdf

Eric Foster had to design a metal detector to find broken needles in spools of fabric before shipping the fabric.

1. First he obtained samples of broken needles and a sample of the fabric.
2. He then calculated the Time Constant of the broken needles to understand how to fully stimulate it and the minimum delay required to detect the broken needles.
3. He then inferred the maximum distance from the coil to the broken needle in the fabric moving under the coil.
4. He then designed a coil as long as the width of the fabric and experimented with the coil width necessary to detect the broken needle as well as the fabric speed to detect the broken needles.

Any unique coil design requires good input data, knowledge of the pulse induction technology theory, prototyping and accurate testing in variable conditions to see where results differ or interact.

I hope this helps?

Joseph J. Rogowski

[GeoMax]

Hi Joseph,

thanks for your reply. You are right, without further details it is difficult to give the right answer.
Actually, my question was more or less a general question/idea whether it would make sense or not before I start building one.

Here some additional details of that I had in mind to build. I have in mind to build a big frame coil of 1x1 meter to detect WWII relicts in about 1 to 1.5 meter depth.
I know that a coil of that size is not very sensitive to small targets like for example coins. Since you carry a big frame over the battlefield, it would be a good idea to
have in a center a smaller coil to catch the small targets in about max. 30cm depth as well. I know from the arrangement you will not cover the same area as the big
coil covers, but at least you cover some area in the middle.

Here some specs of the arrangement.
The TX frequency of the detector is somewhere between 500 to 600Hz, the coil resistance about 2..3 Ohms each, large coil is 1m square, small coil is 0,5m square, coil
inductance needs to be calculated and is yet unknown. Windings of the large coil would be between 12 to 16 windings, the smaller one would have about 20 to 25 windings.
The number of windings are not final but they would be in that range.

No switching of coils. Both coils are in series connected and have the same winding directions. The coil arrangement connects in mono coil configuration to a standard
N-MOSFET detector circuit.

GeoMax

[bbsailor]

Hi Joseph,

thanks for your reply. You are right, without further details it is difficult to give the right answer.
Actually, my question was more or less a general question/idea whether it would make sense or not before I start building one.

Here some additional details of that I had in mind to build. I have in mind to build a big frame coil of 1x1 meter to detect WWII relicts in about 1 to 1.5 meter depth.
I know that a coil of that size is not very sensitive to small targets like for example coins. Since you carry a big frame over the battlefield, it would be a good idea to
have in a center a smaller coil to catch the small targets in about max. 30cm depth as well. I know from the arrangement you will not cover the same area as the big
coil covers, but at least you cover some area in the middle.

Here some specs of the arrangement.
The TX frequency of the detector is somewhere between 500 to 600Hz, the coil resistance about 2..3 Ohms each, large coil is 1m square, small coil is 0,5m square, coil
inductance needs to be calculated and is yet unknown. Windings of the large coil would be between 12 to 16 windings, the smaller one would have about 20 to 25 windings.
The number of windings are not final but they would be in that range.

No switching of coils. Both coils are in series connected and have the same winding directions. The coil arrangement connects in mono coil configuration to a standard
N-MOSFET detector circuit.

GeoMax

GeoMax,

See this link on this forum. There are discussions about very large coils that you should read.

https://www.geotech1.com/forums/show...ighlight=Bombs

Joseph J. Rogowski

[Infamy]

Since this is a topic about large mono coil shapes, the following question fits to this topic.

Usually big frame coils are designed for deep large targets. Those coils are not very sensitive to small targets and usually carried by two people. Right?
Would it make sense to build inside the big coil frame a smaller coil frame to catch additional small targets as well? The coil arrangement would be like
a dual field coil (see image below).

What is your opinion to this idea? Would it make sense?

[ATTACH]57862[/ATTACH]

What if you take the 1mx1m coil and turn it into a 1.5mx0.5m coil, would this make the it more sensitive to smaller targets?
and how would depth be affected?
This is an interesting option for me as I could run a 1m grid and maintain a reasonable overlap, which would be help in adjusting the RTK GPS values as a test target could be set to appear on two different runs this could then be used to average out a more accurate location. Distance from GPS reciever to coil being in the range of 2.5-3m requires adjustment anyway.

I already plan to run a magnetometer and PI detector over the same area, I could add another PI run with a larger or smaller coil. Once the project is built and working, as it runs itself adding another survey on is not a great effort as I don't physical walk the survey. Running different coil sizes may have some use in determining depth and size of target, that would be a useful asset when all the surveys are overlaid in the analysis stage.

[GeoMax]

Hi Joseph,

thanks for the hint on the Geotech post. I have not seen this posts before.

GeoMax

[bbsailor]

Hi Joseph,

thanks for the hint on the Geotech post. I have not seen this posts before.

GeoMax

GeoMax,

To effectively use this forum to find information about your desired topics, use the box located on the upper right side of this forum labeled advanced search. Enter your key word in his box and you will see a list of posts that contain this key word. I used the word bomb to get a large list of forum posts that contain this key word on the link that I provided.

The most important thing to know about pulse induction metal detectors is the theory of how the coil size, inductance, resistance, coil current, TX current rise graph and current discharge slope as related to damping resistor value will all affect the total performance of the PI circuit and coil combination on specific target sizes and detection depth.

Joseph J. Rogowski

[Infamy]

GeoMax,

To effectively use this forum to find information about your desired topics, use the box located on the upper right side of this forum labeled advanced search. Enter your key word in his box and you will see a list of posts that contain this key word. I used the word bomb to get a large list of forum posts that contain this key word on the link that I provided.

The most important thing to know about pulse induction metal detectors is the theory of how the coil size, inductance, resistance, coil current, TX current rise graph and current discharge slope as related to damping resistor value will all affect the total performance of the PI circuit and coil combination on specific target sizes and detection depth.

Joseph J. Rogowski

I was banned from a UK detecting site back in about 2002-2004. They set up a section pushing long range locator crap, the same crap being sold to war zones to detect explosives. I made the point that this garbage technology would get people killed, so they banned me, people did get killed and folks did go to prison. Do you think you can find what I wrote, I'd be interested to see what I actually said. I think I was writing under the name 'Indianas Back' or a variation of that Indianas_Back, IB1.

Thanks Joseph I just don't have your skills or resources.

PS It was a husband and wife run forum probably one of the largest in the UK at the time.

Someone on Minelabowners forum might actually remember the post, perhaps IHG or Sheddy but he's on another site but I forgotton which one.

[Infamy]

GeoMax,


The most important thing to know about pulse induction metal detectors is the theory of how the coil size, inductance, resistance, coil current, TX current rise graph and current discharge slope as related to damping resistor value will all affect the total performance of the PI circuit and coil combination on specific target sizes and detection depth.

Joseph J. Rogowski

PS What about coil shape?

[bbsailor]

I was banned from a UK detecting site back in about 2002-2004. They set up a section pushing long range locator crap, the same crap being sold to war zones to detect explosives. I made the point that this garbage technology would get people killed, so they banned me, people did get killed and folks did go to prison. Do you think you can find what I wrote, I'd be interested to see what I actually said. I think I was writing under the name 'Indianas Back' or a variation of that Indianas_Back, IB1.

Thanks Joseph I just don't have your skills or resources.

PS It was a husband and wife run forum probably one of the largest in the UK at the time.

Someone on Minelabowners forum might actually remember the post, perhaps IHG or Sheddy but he's on another site but I forgotton which one.

Infamy,

Just do a general web search using unique key words. If the web sites that the data you seek is on, has been shut down, you may not find it unless someone has copied it on a still operational web site.

Joseph J. Rogowski

[bbsailor]

PS What about coil shape?

Infamy,

The coil shape is related to the size, metal type and time constant of the target. A large round coil may miss a small target but if that coil is reshaped into a wide and narrow oblong coil, it may detect that small target when that target is near this narrow coil. That is why testing is very important when planning to design coils. Small, low TC targets will have their TX stimulated eddy currents decay by 95 percent in 3 target time constants (TC). That means that the pulse induction delay from TX turn off to RX turn on needs to be be very low for low TC targets. This variable can be optimized by coil size, shape and coil construction technique.

Joseph J. Rogowski

[Carl-NC]

PS What about coil shape?

Round is most efficient, anything else reduces efficiency but can have other benefits. Ferinstance, a 24"x12" elliptical coil will have raw depth close to a 15" round coil, but it will have better coverage and the tighter radii at the toe & heel will improve sensitivity to smaller targets.

It sounds like you are building a sled coil to pull over the water and swamp. Depending on what you are hunting, I'd probably go with a 2:1 or 3:1 sideways rectangle for coverage. Coil size depends on the intended target; if you are looking for Atocha bars @ 2m deep then 1x3 meters might be good. Tons of variables.